As described in previous entries using markers to perform a tracking presents more disadvantages than using the object itself as a target to be tracked. Some of those disadvantages are the need to print the marker or that the tracking can fail due to occlusions. Also, these markers are invasive to the environment, using marketing expression, “do not keep the packaging clean”.
Due to these reasons, many researchers and companies have focused on developing markerless tracking systems instead of marker-based tracking systems. The former will be the subject of this entry.
Figure 1. Online Monocular techniques scheme.
Techniques developed for online monocular markerless augmented reality systems can be classified into two sub-branches: model based and Structure from Motion (SfM) based. The difference is that while in the former a previous knowledge about the real world before the tracking is performed is required, in the later this knowledge is acquired during the tracking. Inside these two sub-branches two different approaches can be taken into consideration according the nature of the tracking. The first of them, known as recursive tracking, uses the previous known pose to estimate the current one. The second option, which is called tracking by detection, allows to calculate the pose estimation without any previous knowledge or estimation, which can be better for recovering from failures.
Furthermore, the model based approaches which use a recursive tracking can be classified in three branches or categories: edge based, optical flow based and textured based. In the other hand, the approaches covered by tracking by detection techniques are: edge based techniques and texture-based techniques. Although techniques based on tracking by detection seem to be a better option, several things have to be taken into consideration before to choose any of them in order to select the option which fits our requirements, like frame rate, accuracy or even object tracked.